Contamination of silicon wafers with heavy metals such as Fe, Mg, Ca, Ni, Cu, etc. may result in depressed yield and in reduced reliability. The heavy metal atoms may be incorporated in the transistor gate dielectric during growth resulting in poor gate oxide integrity or in premature wearout of the gate dielectric. Heavy metal ions may also prevent silicon defects from healing during anneal steps resulting in increased diode leakage. One method of removing heavy metal contaminants from the critical active areas near the surface of integrated circuit wafers is to grow bulk micro defects (BMD) beneath a denuded surface layer. Heavy metal atoms diffuse to and get trapped (gettered) by the BMDs thus removing them from regions where they may cause harm.
BMD's nucleate and grow during thermal steps in integrated circuit process flows. Here to fore semiconductor process flows have sufficient thermal budget (temperature multiplied by time at temperature) in the process flow to nucleate and grow BMDs for efficient gettering. Recently, the thermal budget of some ultra deep submicron process flows is no longer sufficient to grow microdefects (BMDs) of an adequate concentration and size for effective gettering. Poor gettering may result in depressed yield. The wafer shown in FIG. 1 has poor yield at the edges of the wafer, 1002, as a result of poor gettering in the edge of the wafer.